CN109643590B

CN109643590B - Conductive paste and wiring board using the same

Info

Publication number: CN109643590B
Application number: CN201780052618.4A
Authority: CN
Inventors: 山田牧; 胜又梨江; 青山征人
Original assignee: Yazaki Corp
Current assignee: Yazaki Corp
Priority date: 2016-09-21
Filing date: 2017-06-12
Publication date: 2020-06-23
Anticipated expiration: 2037-06-12
Also published as: US20190185684A1; DE112017004749T5; JP6574746B2; JP2018049735A; WO2018055848A1; CN109643590A

Abstract

A conductive paste includes metal nanoparticles protected by an organic compound including an amino group and having an average particle diameter of 30nm to 400nm, metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm, an organic solvent, and a resin component, a conductor obtained by firing the conductive paste has a film thickness of 30 μm or more and a film thickness of 5.0 × 10^‑6Specific resistance of not more than Ω · cm. In this way, the conductive paste can reduce the resistance of the resulting conductor and can increase the amount of current flowing. A wiring board includes a conductor obtained from the conductive paste.

Description

Conductive paste and wiring board using the same

Technical Field

The present invention relates to a conductive paste and a wiring board using the same. More particularly, the present invention relates to a conductive paste that can provide a conductor having a specific resistance equivalent to the specific resistance of a silver block, and a wiring board using the conductive paste.

Background

In recent years, due to a reduction in cable layout space of automobiles, there is a demand for a flexible printed wiring board capable of realizing miniaturization, thinning, three-dimensional formation, and the like of a wire harness and peripheral components thereof. In particular, the map lamp disposed in the vicinity of the rear view mirror and at the front center of the passenger compartment is required to be thinned. In other words, with the development of autobrake vehicles and autodrive vehicles, the functions of cameras and sensor modules are enhanced, and it is required to mount these components behind map lamps, so that it is necessary to reduce the thickness of the map lamps. Therefore, in order to reduce the thickness of the map lamp, the need for the flexible printed wiring board as described above is increased.

As a flexible printed wiring board that satisfies the requirements of miniaturization, thinning, three-dimensional formation, and the like, there is known a Flexible Printed Circuit (FPC) in which a circuit is formed on a base material obtained by bonding a thin soft base film having electrical insulation properties and a conductive metal such as a copper foil. The circuit of the FPC is generally manufactured by a method called a subtractive method. For example, a circuit can be formed by bonding a metal foil such as a copper foil to a polyimide film and etching the metal foil. Such subtractive methods require complicated and extremely long processes such as photolithography, etching and chemical vapor deposition, and involve problems of low yield. Further, in processes such as photolithography and etching, environmental issues such as waste liquid are generally regarded as problems.

In order to solve the above problems, an additive method of forming a conductor pattern on an insulating plate, which is opposite to the subtractive method, has been studied. The method comprises a plurality of types of methods, and mainly comprises the following steps: plating, printing of conductive paste, etc., vapor deposition of metal on a desired portion of a substrate, arrangement of a polyimide-coated cable by adhesion to a substrate, adhesion of a pre-formed pattern to a substrate, etc.

Among these additive methods, the printing method is one of the methods with the highest productivity. In the printing method, a circuit is constructed mainly by using a film as a base material and also using a conductive ink or a conductive paste as a wire, and bonding an insulating film, a resist, or the like thereto. Such conductive ink or conductive paste is composed of a metal component, an organic solvent, a reducing agent, a binder, and the like, and forms a conductor by firing after coating and enables conduction.

As the conductive paste, there are a conductive paste using metal nanoparticles having a particle diameter of less than 1 μm as a main component and a conductive paste using metal nanoparticles having a particle diameter of more than 1 μm as a main component. However, when metal nanoparticles are used as the main component, the thickness of the coating film of the conductive paste cannot be maintained, and as a result, the resulting conductor will become a thin film after firing. In addition, when metal nanoparticles are used as the main component, it is difficult to form a dense sintered body even if the conductive paste is fired, so the specific resistance of the resulting conductor will increase. Therefore, the conventional conductive pastes cannot increase the amount of current flowing through the conductors, and it is difficult to apply them to wiring boards for automobiles. Therefore, conductive pastes using metal nanoparticles having a particle size of less than 1 μm and metal nanoparticles having a particle size of more than 1 μm have been developed.

As such a conductive paste, patent document 1 discloses a conductive paste including: (A) a flake-like silver powder having an average particle diameter of 2 to 20 μm, a predetermined tap density, and a content ratio of a carbon-containing compound of 0.5 mass% or less; (B) silver nanoparticles having an average particle diameter of 10 to 500 nm; and (C) a thermosetting resin. Further, patent document 2 discloses a conductive metal paste using ultrafine metal particles having an average particle diameter of 100nm or less, the surfaces of the ultrafine metal particles being covered with a predetermined compound; and a metal filler having an average particle diameter of 0.5 to 20 μm, and includes a resin component to be cured by heating, an organic acid anhydride or an organic acid, and an organic solvent. Patent document 3 discloses a conductive ink including: fine metal particles (a) having an average particle diameter of 0.001 to 0.1 μm; a foil-like metal powder (B) having an average circle-equivalent diameter of 1 to 20 μm and an average thickness of 0.01 to 0.5 μm; and a resin. Patent document 4 discloses a low-temperature-fired silver paint including at least: metal nanoparticles covered with an organic protective colloid; a silver filler; and a dispersion medium, wherein the organic protective colloid and the dispersion medium have a decomposition temperature or boiling point of 70 to 250 ℃. Patent document 5 discloses a conductive paste for screen printing, which includes: metal nanoparticles protected by an organic compound including a basic nitrogen atom and having an average particle diameter of 1 to 50 nm; metal particles having an average particle size of more than 100nm up to 5 μm; a deprotecting agent for the metal nanoparticles; and an organic solvent. Patent document 6 discloses a silver particle-coated compound including: silver nanoparticles, the surface of which is covered with a protective agent comprising an aliphatic hydrocarbon amine; vinyl chloride-vinyl acetate copolymer resin; and a dispersion solvent.

Reference list

Patent document

Patent document 1: japanese unexamined patent application publication No. JP 2015-162392A

Patent document 2: international publication No. WO 2002/035554

Patent document 3: japanese unexamined patent application publication No. JP 2005-248061A

Patent document 4: japanese unexamined patent application publication No. JP 2008-91250A

Patent document 5: japanese patent publication No. JP 4835810B 2

Patent document 6: international publication No. WO 2016/052033

Disclosure of Invention

However, even if the conductive paste of patent documents 1 to 6 is used, the resulting conductor still has a high specific resistance. Therefore, since the amount of current that can pass through the conductors is small, it is difficult to use them as wiring boards for automobiles.

The present invention has been made in view of the problems of such conventional techniques. An object of the present invention is to provide a conductive paste capable of reducing the resistance of a resultant conductor and increasing the film thickness thereof, and capable of increasing the amount of current flowing through the conductor, and a wiring board using the conductive paste.

A conductive paste according to a first aspect of the invention includes metal nanoparticles protected by an organic compound containing an amino group and having an average particle diameter of 30nm to 400nm, metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm, an organic solvent, and a resin component composed of a cellulose derivative, a conductor obtained by firing the conductive paste has a film thickness of 30 μm or more and a film thickness of 5.0 × 10^-6Specific resistance of not more than Ω · cm.

A conductive paste according to a second aspect of the invention relates to the conductive paste according to the first aspect, wherein the organic compound is an aliphatic hydrocarbon amine having: an aliphatic hydrocarbon group which is a linear or branched alkyl group having 4 to 16 carbon atoms in total; and one or two amino groups.

The electrically conductive paste according to a third aspect of the invention relates to the electrically conductive paste according to the first or second aspect, wherein the higher fatty acid is at least one of a saturated fatty acid and an unsaturated fatty acid each having a total of 12 to 24 carbon atoms.

A conductive paste according to a fourth aspect of the invention relates to the conductive paste according to any one of the first to third aspects, wherein the metal nanoparticles have an average particle diameter of 70nm to 310nm, and the metal particles have an average particle diameter of 1 μm to 3 μm.

A conductive paste according to a fifth aspect of the present invention relates to the conductive paste according to any one of the first to fourth aspects, wherein the organic solvent has a total of 8 to 16 carbon atoms, has a hydroxyl group, and further has a boiling point of 280 ℃ or less.

A conductive paste according to a sixth aspect of the invention relates to the conductive paste according to any one of the first to fifth aspects, wherein the resin component is made of a thermoplastic resin.

A wiring board according to a seventh aspect of the present invention includes a conductor obtained by the conductive paste according to any one of the first to sixth aspects.

Detailed Description

[ conductive paste ]

The conductive paste according to the embodiment includes: metal nanoparticles protected by an organic compound including an amino group and having an average particle diameter of 30nm to 400 nm; and metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm.

The metal nanoparticles in the present embodiment have an average particle diameter of 30nm to 400 nm. Generally, as the diameter of the metal particles decreases, the number of metal atoms present on the surface of the particles increases, so that the melting point of the metal decreases. Therefore, such metal nanoparticles are used in the conductive paste, thereby enabling the conductor to be formed at a lower temperature. Further, since the average particle diameter of the metal nanoparticles is 30nm to 400nm, the gaps between the respective metal particles can be filled with the metal nanoparticles. Thus, by firing the conductive paste, the metal nanoparticles and the metal particles are sintered to form a dense sintered body, so that the conductivity of the resulting conductor can be improved. Incidentally, the average particle diameter of the metal nanoparticles is more preferably 70nm to 310nm from the viewpoint of forming a denser sintered body and improving conductivity. As used herein, the average particle size of the metal nanoparticles refers to the median diameter (50% diameter, D50) measured by dynamic light scattering method.

The metal of the metal nanoparticles constituting preferably contains at least one element selected from the group consisting of gold, silver, copper, and platinum, and more preferably, is constituted of at least one element selected from the group consisting of gold, silver, copper, and platinum. By using metal nanoparticles composed of these metals, thin wires can be formed. Further, the resistance value of the conductor after firing can be reduced, and the surface smoothness of the conductor can also be improved. Among these metals, silver is preferably used from the viewpoint of facilitating the formation of a dense sintered body by firing of the conductive paste, so that the specific resistance of the resulting conductor can be reduced.

Due to metal nanoparticlesThe surface energy increases as a result of the refinement, so aggregation and precipitation of metal nanoparticles easily occur. Therefore, in order to suppress aggregation and precipitation of the metal nanoparticles, the surface of the metal nanoparticles is protected by an organic compound containing an amino group (-NH)₂). As such an organic compound, it is more preferable to use an aliphatic hydrocarbon amine having: an aliphatic hydrocarbon group which is a linear or branched alkyl group having a total of 4 to 16 carbon atoms; and one or two amino groups. These amine compounds can be easily removed through the firing process while maintaining the highly dispersed state of the metal nanoparticles, and thus low-temperature firing of the metal nanoparticles can be promoted. As such an organic compound, at least one selected from the group consisting of n-butylamine, n-hexylamine, and n-octylamine can be used. The amount of the organic compound to be added is preferably 1 to 3 moles per mole of the metal nanoparticles from the viewpoint of suppressing aggregation of the metal nanoparticles.

The conductive paste according to the present embodiment includes, in addition to the above-described metal nanoparticles, metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm. By using such metal particles, the conductor after firing can be made dense, and the specific resistance can be reduced. Further, by using the metal nanoparticles and the metal particles in combination, the thickness of the resulting conductor can be increased.

The average particle diameter of the metal particles is preferably 1 μm to 5 μm. When the average particle diameter of the metal particles falls within this range, it becomes possible to thicken the resulting conductor, thereby increasing the conductivity of the conductor. Even when a conductive paste is applied to an insulating substrate by a screen printing method to be described later, there is little possibility that metal particles are clogged in the screen printing mesh, so that a fine circuit can be efficiently formed. In addition, the metal particles more preferably have an average particle diameter of 1 μm to 3 μm from the viewpoint of forming a dense sintered body together with the metal nanoparticles and improving conductivity. As used herein, the average particle diameter of the metal particles refers to the median diameter (50% diameter, D50) measured by dynamic light scattering.

With respect to the metal nanoparticles, the metal constituting the metal particles preferably contains at least one element selected from the group consisting of gold, silver, copper, and platinum, and more preferably, is composed of at least one element selected from the group consisting of gold, silver, copper, and platinum. By using metal particles composed of these metals, the resistance value of the conductor after firing can be reduced, and the surface smoothness of the conductor can also be improved. Among these metals, silver is preferably used from the viewpoint of facilitating the formation of a dense sintered body by firing of the conductive paste, so that the specific resistance of the resulting conductor can be reduced.

The metal particles have a lower surface energy than the metal nanoparticles, and aggregation and precipitation of the metal particles are less likely to occur. However, from the viewpoint of suppressing aggregation and precipitation of the metal particles, the surfaces of the metal particles are protected by the higher fatty acid. Preferably, the higher fatty acid is at least one of saturated fatty acid and unsaturated fatty acid each having a total number of 12 to 24 carbon atoms. Specifically, as the higher fatty acid, at least one selected from the group consisting of myristic acid, palmitic acid, stearic acid, oleic acid, and linoleic acid, and derivatives thereof can be used. The amount of the higher fatty acid to be added is preferably 1 to 3 moles per mole of the metal particles from the viewpoint of suppressing aggregation of the metal particles.

In the conductive paste of the present embodiment, the ratio between the metal nanoparticles and the metal particles is not particularly limited, but is preferably, for example, 3: 7 to 7: 3 in a weight ratio. When the ratio between the metal nanoparticles and the metal particles falls within this range, a conductor composed of a dense sintered body and having improved conductivity can be obtained. If the proportion of the metal nanoparticles is lower than this range, it may be difficult to satisfy the specific resistance of the resulting conductor. On the contrary, when the proportion of the metal nanoparticles is higher than this range, there is a possibility that the viscosity of the conductive paste may be lowered, thereby making it difficult to satisfy the workability.

The conductive paste of the present embodiment contains an organic solvent in order to highly disperse the metal nanoparticles protected by the amino organic compound and the metal particles protected by the higher fatty acid. The organic solvent is not particularly limited as long as it can highly disperse the metal nanoparticles and the metal particles and also can melt a resin component which will be described later. As the organic solvent, an organic solvent having a total number of carbon atoms of 8 to 16, having a hydroxyl group, and also having a boiling point of 280 ℃ or less is preferably used. Specifically, as the organic solvent, at least one selected from the group consisting of terpineol (C10, boiling point: 219 ℃ C.), dihydroterpineol (C10, boiling point: 220 ℃ C.), alcohol ester dodeca (C12, boiling point: 260 ℃ C.), 2, 4-dimethyl-1, 5-pentanediol (C9, boiling point: 150 ℃ C.), and butyl carbitol (C8, boiling point: 230 ℃ C.) can be used. Further, as the organic solvent, at least one selected from the group consisting of isophorone (boiling point: 215 ℃), ethylene glycol (boiling point: 197 ℃), diethylene glycol butyl ether acetate (boiling point: 247 ℃), and 2,2, 4-trimethyl-1, 3-pentanediol diisobutyrate (C16, boiling point: 280 ℃) can be used.

The amount of the organic solvent to be added in the conductive paste is not particularly limited, but is preferably adjusted so that the conductive paste obtains a viscosity that allows it to be applied by a screen printing method or the like. Specifically, the amount of the organic solvent to be added is preferably 2 to 10 parts by mass, more preferably 3 to 8 parts by mass, based on 100 parts by mass of the total of the metal nanoparticles protected by the organic compound containing an amino group and the metal particles protected by the higher fatty acid.

The conductive paste of the present embodiment contains a resin component in order to increase the film thickness of the conductor to be obtained and to reduce the specific resistance. By adding the resin component, it becomes possible to make the coating film of the conductive paste thicker, and to increase the film thickness of the conductor obtained after firing to 30 μm or more.

The resin component is preferably a thermoplastic resin. Examples of the thermoplastic resin include polyolefin resins, polyamide resins, elastomer (styrene, olefin, polyvinyl chloride (PVC), ester and amide) resins, acrylic resins, and polyester resins. Specific examples of the thermoplastic resin include engineering plastics, polyethylene, polypropylene, nylon resins, acrylonitrile-butadiene-styrene (ABS) resins, acrylic resins, ethylene vinyl acetate resins, and polystyrene resins. Further, polyphenylene sulfide resin, polycarbonate resin, polyester elastomer resin, polyamide elastomer resin, liquid crystal polymer, polybutylene terephthalate resin, and the like can also be specified. One of these thermoplastic resins may be used alone, or two or more thereof may be used in combination.

As the resin component, a fiber-forming chain polymer material (fiber type resin), for example, a cellulose derivative, is preferably used. Examples of the cellulose derivative include cellulose ether, cellulose ester and cellulose ether ester, but preferably cellulose ether is used. Cellulose ethers include cellulose monoethers in which one ether group is bonded to cellulose and cellulose mixed ethers in which two or more ether groups are bonded to cellulose. Specific examples of the cellulose monoethers include methyl cellulose, ethyl cellulose, propyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, and carboxymethyl cellulose. Specific examples of the cellulose mixed ether include methylethylcellulose, methylpropylcellulose, ethylpropylcellulose, hydroxymethylethylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, and hydroxypropylmethylcellulose. One of these cellulose ethers may be used alone, or two or more thereof may be used in combination.

The amount of the resin component to be added in the conductive paste is not particularly limited, but is preferably adjusted so that the thickness of the coating film of the conductive paste can be increased. Specifically, the amount of the resin component is preferably 0.1 to 5 parts by mass based on 100 parts by mass of the total of the metal nanoparticles protected by the organic compound containing an amino group and the metal particles protected by the higher fatty acid.

The conductive paste of the present embodiment can contain an additive that improves printing characteristics and conductor characteristics, such as an antifoaming agent, a surfactant, or a rheology control agent, within a range that does not adversely affect the dispersion stability of the paste and the properties of the conductor after firing.

Next, a method of preparing the conductive paste of the present embodiment will be described. First, metal nanoparticles protected by an organic compound including an amino group are prepared. The method of preparing the metal nanoparticles protected by the capping agent is not particularly limited, and the metal nanoparticles can be obtained by, for example, directly mixing the metal nanoparticles in powder form with an organic compound. Alternatively, the metal nanoparticles are obtained by mixing metal nanoparticles in powder form with an organic compound using an organic solvent, and then drying the mixture. The drying method is also not particularly limited, and the organic solvent can be removed by vacuum drying or freeze drying.

Likewise, metal particles protected by higher fatty acids were prepared. The method of producing the metal particles protected by the covering agent is also not particularly limited, and the metal particles can be obtained by, for example, directly mixing the metal particles in powder form with a higher fatty acid. It is also possible to obtain the metal particles by mixing the metal particles in powder form with a higher fatty acid using an organic solvent and then drying the mixture.

Then, the metal nanoparticles protected by the organic compound containing an amino group, the metal particles protected by the higher fatty acid, the organic solvent, the resin component, and if necessary, the additive are mixed. The mixing method is not particularly limited, and it is preferable to mix the components using, for example, a spin/spin centrifuge. In addition, when necessary, defoaming treatment of the obtained mixture was performed. Through such a process, the conductive paste of the present embodiment can be obtained.

As described above, the conductive paste of the present embodiment comprises metal nanoparticles protected by an organic compound containing an amino group and having an average particle diameter of 30nm to 400nm, metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm, an organic solvent, and a resin component, and since a conductor obtained by firing the conductive paste becomes a dense sintered body having an increased film thickness, the amount of flowing current can be increased^-6Specific resistance of omega cm or less, and thus has the same specific resistance as silver block, and can be applied for use in vaporA wiring board of the vehicle.

[ Wiring Board ]

The wiring board according to the present embodiment includes the conductor obtained from the above-described conductive paste, as described above, the conductor obtained from the conductive paste of the present embodiment has a film thickness of 30 μm or more and 5.0 × 10^-6Specific resistance of not more than Ω · cm. Therefore, the amount of current flowing can be increased, and the resulting wiring board can be suitably used for automobiles.

The wiring board of the present embodiment can be obtained by applying the conductive paste to the base material in a desired shape and then firing. The substrate that can be used for the wiring board is not particularly limited, and an electrically insulating film or sheet material can be used. Such a substrate has flexibility and can be folded according to the use position. The material of the base material is not particularly limited, and may be at least one selected from the group consisting of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Polycarbonate (PC), polypropylene (PP), and polybutylene terephthalate (PBT).

The method of applying the conductive paste to the substrate is not particularly limited, and the conductive paste can be applied to the substrate by a conventionally known method, such as flexography, gravure offset, screen printing, or rotary screen printing.

The firing method after applying the conductive paste to the substrate is also not particularly limited. For example, it is preferable that the substrate to which the conductive paste has been applied is exposed to hot air at 150 ℃ or higher. As a result, the organic compound, higher fatty acid, organic solvent and resin component in the conductive paste are removed, and the metal nanoparticles and the metal particles are sintered, so that a conductor with high conductivity can be obtained. More preferably, the substrate to which the conductive paste has been applied is exposed to hot air at a temperature of 250 ℃ or higher. By increasing the firing temperature, the obtained sintered body becomes dense, and thus the resistance can be further reduced. Incidentally, the firing method is not limited to the above-described hot air firing, and for example, plasma firing, photo-firing, or pulse firing can also be applied.

The wiring board provided with the conductor obtained by the conductive paste may be provided with an insulating covering material for covering and protecting the surface of the conductor. As the insulating cover material, an insulating film or a resist can be used. Preferably, the insulating cover material is made of polyethylene terephthalate (PET), polyethylene naphthalate (PEN), Polycarbonate (PC), polypropylene (PP), polybutylene terephthalate (PBT), Polyurethane (PU), or the like, having an adhesive on one side. The resist to be used is preferably a thermosetting resist or a UV-curable resist, and particularly preferably an epoxy resist or a polyurethane resist.

Examples

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[ preparation of sample ]

First, as shown in tables 1 to 3, silver nanoparticles having a median diameter of 30nm, 70nm, 150nm, 240nm, 310nm or 600nm were mixed with n-hexylamine or n-butylamine, thereby obtaining alkylamine-protected silver nanoparticles. The mass ratio of the silver nanoparticles to n-hexylamine or n-butylamine was set to 1: 1. silver nanoparticles protected by alkylamine were obtained by mixing silver nanoparticles having a median diameter of 310nm with n-hexylamine and n-butylamine. Setting the mass ratio of the silver nanoparticles to the n-hexylamine to the n-butylamine to be 1: 0.5: 0.5.

further, as shown in tables 1 to 3, silver particles having a median diameter of 0.8 μm, 1 μm, 2.3 μm, 2.9 μm, 5 μm or 7 μm were mixed with stearic acid or oleic acid, thereby obtaining silver particles protected with a higher fatty acid. The mass ratio between the silver particles and the stearic acid or oleic acid was set to 1: 1. also, silver particles having a median diameter of 1.8 μm and not treated with a higher fatty acid were prepared.

Then, the silver nanoparticles and silver particles obtained as described above, and the organic solvents and resin components as shown in tables 1 to 3 were stirred using a spin/spin centrifuge at the ratios as shown in the respective tables to prepare conductive pastes of the respective examples. In addition, the following organic solvents and resin components were used.

(organic solvent)

The alcohol ester dodeca (2,2, 4-trimethylpentane-1, 3-diol monoisobutyrate) manufactured by Istman chemical company

Terpineol (2- (4-methylcyclohexylamine-3-enyl) isopropanol) produced by Dioscorea panthaica Misch products industries, Ltd

Cyclohexane manufactured by Ed.Meco.Ltd

Methyl ethyl ketone manufactured by Wanshan petrochemical Co., Ltd

(resin component)

Ethyl cellulose manufactured by dow chemical corporation, ETHOCEL (registered trademark) STD10

Epoxy resin manufactured by Mitsubishi chemical corporation, JeR (registered trademark) host 828/curing agent ST11

Polyurethane resin manufactured by Mitsukawa chemical industry Co., Ltd, UREARNO (registered trademark)

[ Table 1]

[ Table 2]

[ Table 3]

[ evaluation ]

The film thickness and specific resistance of each conductor obtained by firing the conductive pastes of examples 1 to 12 and comparative examples 1 to 5 obtained as described above, and the viscosity of each conductive paste and the appearance thereof at the time of coating were evaluated as follows. The evaluation results are collectively shown in tables 1 to 3.

(film thickness of conductor)

The film thickness of each conductor was measured by a stylus scanning method by referring to japanese industrial standard JIS H8501 (thickness measuring method for metal coating). As an apparatus, a contact-type film thickness measuring apparatus (Alpha-Step D-500 manufactured by KLA Tencor) was used.

Specifically, first, the respective circuits of the conductive paste having a width of 1mm and a length of 10cm and the respective circuits of the conductive paste having a width of 5mm and a length of 10cm were printed on the polyimide substrate by using a screen printer. The substrate on which the conductive paste was printed was left at room temperature for 30 minutes, and then fired with hot air at 150 ℃ for 30 minutes to prepare samples of each example.

Next, the film thickness of the silver thin film of the obtained sample was measured at three points, i.e., at a portion 1cm from each end and at a portion 5cm from the center. The needle speed at the time of measurement was 0.1mm/s and the stylus pressure was 15 mg. The needle is moved in a direction perpendicular to the circuit to take a measurement. The average value of the film thicknesses at the three positions was used as the evaluation result of each example.

(specific resistance of conductor)

The specific resistance of each conductor was measured with reference to JIS K7194 (measurement method of resistivity of conductive plastic having a four-point probe array). As the apparatus, a four-point probe resistivity measuring apparatus (resistivity measuring apparatus Sigma-5+ manufactured by NPS corporation) was used.

Specifically, first, each of the conductive paste-coated circuits having a width of 2mm and a length of 10cm was printed on the polyimide substrate by using a screen printer. The substrate on which the conductive paste was printed was left at room temperature for 30 minutes, and then fired with hot air at 150 ℃ for 30 minutes to prepare samples of each example.

Then, with respect to the silver thin film on the obtained sample, surface resistances were measured at three points, i.e., at a portion 1cm from each end and at a portion 5cm from the center. The surface resistance was measured in a state where the needle was placed parallel to the circuit.

(viscosity of conductive paste)

The viscosity of each conductive paste was measured with reference to JIS K5600-2-3 (test method of coating material-part 2: characteristics and stability of coating material, and part 3: viscosity (cone plate method)). As the apparatus, a rotational viscometer (rheometer RS100-CS manufactured by HAKKE) was used.

Specifically, first, after the conductive paste of each example was prepared, it was left at room temperature (25 ℃) to keep the temperature constant. A temperature controller was also used during the viscosity measurement to control the temperature of the conductive paste at 25 ℃. Then, the conductive paste was filled between the cone and the plate at the measuring portion, rotated so as to obtain a prescribed shear rate, and the viscosity at that time was measured. At this time, the shear rate was changed from 0S within 5 minutes^-1Change to 100S^-1While measuring the viscosity, and using the shear rate of 10S^-1The value of (A) was taken as the viscosity of each example.

(appearance of conductive paste at the time of coating)

When the film thickness of the above conductor was measured, after screen printing, it was visually confirmed whether or not the conductive paste was uniformly applied without rubbing against the conductive paste, and then, the case where no rubbing against the conductive paste was applied and a uniform coating was applied was evaluated as "○ (good)".

As shown in tables 1 and 2, in the conductive pastes according to examples 1 to 12 of this embodiment mode, the film thickness of each conductor obtained was 30 μm or more and the specific resistance was 5.0 × 10^-6Omega cm or less. Therefore, the conductor can be suitably used for a wiring board of an automobile.

In contrast, as shown in table 3, since the conductive paste of comparative example 1 does not contain silver particles, it is difficult to increase the film thickness of the conductive paste, and screen printing cannot be performed. In addition, since the average particle diameter of the silver nanoparticles exceeds 400nm, the conductive paste of comparative example 2 causes deterioration of specific resistance. It can be presumed that a dense conductor cannot be formed because the silver nanoparticles are too large and cannot be filled in the gaps between the silver particles.

In the conductive paste of comparative example 3, since the average particle diameter of the silver particles is less than 1 μm, the specific resistance is deteriorated. It can be presumed that even if silver nanoparticles are used, the silver particles are too small to form a dense conductor. In the conductive paste of comparative example 4, since the average particle diameter of the silver particles is more than 5 μm, the specific resistance is deteriorated. Therefore, it is understood that the average particle diameter of the metal particles is preferably 5 μm or less.

In the conductive paste of comparative example 5, the specific resistance was deteriorated since the surface of the silver particles was not protected by the higher fatty acid. It can be presumed that even if silver nanoparticles are used, the silver particles aggregate because there is no protective agent on the surface of the silver particles, and a dense conductor cannot be formed.

Although the present invention has been described with reference to the embodiments and the comparative examples, the present invention is not limited thereto, and various modifications can be made within the scope of the gist of the present invention.

Japanese patent application No.2016-184242 (filing date: 2016, 9, 21) is hereby incorporated by reference in its entirety.

Industrial applicability

According to the conductive paste of the present invention, the conductor obtained by firing has a film thickness of 30 μm or more and 5.0 × 10^-6Specific resistance of not more than Ω · cm. Therefore, the resistance of the resulting conductor can be reduced and the amount of current flowing can be increased.

Claims

1. An electrically conductive paste comprising:

metal nanoparticles protected by an organic compound including an amino group and having an average particle diameter of 30nm to 400 nm;

metal particles protected by a higher fatty acid and having an average particle diameter of 1 μm to 5 μm;

an organic solvent; and

a resin component composed of a cellulose derivative,

wherein a conductor obtained by firing the conductive paste at 150 ℃ has a film thickness of 30 μm or more and 5.0 × 10^-6Specific resistance of not more than Ω · cm.

2. The conductive paste according to claim 1, wherein the organic compound is an aliphatic hydrocarbon amine having: an aliphatic hydrocarbon group which is a linear or branched alkyl group having a total of 4 to 16 carbon atoms; and one or two amino groups.

3. The electroconductive paste according to claim 1 or 2, wherein the higher fatty acid is at least one of a saturated fatty acid and an unsaturated fatty acid each having a total of 12 to 24 carbon atoms.

4. The conductive paste according to claim 1 or 2, wherein the metal nanoparticles have an average particle diameter of 70nm to 310nm, and the metal particles have an average particle diameter of 1 μm to 3 μm.

5. The conductive paste according to claim 1 or 2, wherein the organic solvent has a total of 8 to 16 carbon atoms, has a hydroxyl group, and further has a boiling point of 280 ℃ or less.

6. A wiring board comprising a conductor obtained from the conductive paste according to any one of claims 1 to 5.